Plasticity in an Electrosensory system II: Postsynaptic Events Associated
With a Dynamic Sensory Filter.
Bastian, Joseph.
Department of Zoology, University of Oklahoma, Norman, OK 73019, USA.APStracts 3:0104N, 1996.
SUMMARY AND CONCLUSIONS
1. This report summarizes studies of the changes in postsynaptic potentials
that occur as pyramidal cells within the primary electrosensory processing
nucleus learn to reject repetitive patterns of afferent input. The rejection
mechanism employs "negative image inputs" which oppose or cancel
electroreceptor afferent inputs or patterns of pyramidal hyperpolarization or
depolarization caused by intracellular current injection. Feedback pathways
carrying descending electrosensory, as well as other types of information
provide the negative image inputs. This study focuses on the role of a
directly descending projection from a second-order electrosensory nucleus, the
n. praeeminentialis (nP), which provides excitatory and inhibitory inputs to
the apical dendrites of electrosensory lateral line lobe (ELL) pyramidal
cells. 2. Electrical stimulation of the pathway linking the nP to the ELL was
used to activate descending inputs to the pyramidal cells. Pyramidal cell
activity was typically increased due to stimulation of this pathway. Tetanic
stimulation of the descending pathway paired with either electrosensory
stimuli which inhibited pyramidal cells, or hyperpolarizing current injection,
increased the excitation provided by subsequent stimulation of this pathway.
Pairing tetanic stimulation with excitatory electrosensory stimuli, or
depolarizing current injection, had the opposite effect. Subsequent activation
of the descending pathway inhibited pyramidal cells. 3. Intracellular
recordings showed that the increased firing of pyramidal cells evoked by
stimulation of the descending pathway following tetanic stimulation paired
with postsynaptic hyperpolarization resulted from larger amplitude and longer
duration EPSPs. The shift in the effect of activity in this descending pathway
to providing net inhibitory input to the pyramidal cells following paired
presynaptic activity and postsynaptic depolarization probably results from the
potentiation of IPSPs. The EPSP and IPSPs evoked by activity in this
descending pathway can be continuously adjusted in amplitude thereby
counterbalancing patterns of pyramidal cell excitation and inhibition received
from the periphery with the result that repetitive patterns of afferent
activity are strongly attenuated.

Received 25 March 1995; accepted in final form 21 May 1996.
APS Manuscript Number J244-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 5 June 96